1. Field of the Invention
The present invention relates to a solid-state image sensing device, a driving method thereof, and an image scanner. More particularly, the present invention relates to a solid-state image sensing device which has a plurality of groups of sensors having different reading periods of signal charge from a pixel line, a driving method thereof, and an image scanner to which the solid-state image sensing device is applied.
2. Description of the Related Art
CCD (Charge Coupled Device) linear sensors, which are composed of solid-state image sensing devices arranged in a one-dimensional array, have been used as image sensors of image scanners in image-input devices such as digital-color copying machines and facsimiles, and as image sensors of image scanners in order to input images for display by personal computers and so on.
Here, when giving an example of a case where the CCD linear sensor is used as an image sensor of an image-input device of digital-color copying machine, for color documents, the image sensor performs image sensing relatively slowly in order to increase color reproduction, whereas, for monochrome documents, the image sensor performs high-speed image sensing in order to increase the copying speed. In such a case, a plurality of groups of sensors which have different transfer speeds are arranged on the same chip.
Specifically, as shown in FIG. 4, a well-known CCD linear sensor has a structure in which, as a monochrome sensor 100, one pixel line (sensor line) 101 having transfer registers 102o and 102e at both sides thereof is arranged, and as a color sensor 200, individual pixel lines 210R, 201G, and 201B, corresponding to R (red), G (green), and B (blue), respectively, having transfer registers 202R, 202G, and 202B, respectively, are arranged.
In the monochrome sensor 100, between the pixel line 101 and two transfer registers 102o and 102e, there is a read-out gate 103o, which reads the signal charge from the odd-numbered pixels in the pixel line 101 to one of the transfer registers 102o, and there is a read-out gate 103e, which reads the signal charge from the even-numbered pixels in the pixel line 101 to one of the transfer registers 102e. Also, output parts 104o and 104e and output circuits 105o and 105e are arranged at the respective output sides of the transfer registers 102o and 102e. 
In the color sensor 200, between each of the pixel lines 201R for R, 201G for G, and 202B for B, and the transfer registers 202R, 202G, and 202B, respectively, there are read-out gates 203R, 203G, and 203B, which read the signal charge from the pixels of the pixel lines 201R, 201G, and 201B to one of the transfer registers, 202R, 202G, and 202B, respectively. Also, output parts 204R, 204G, and 204b, and output circuits 205R, 205G, and 205B are arranged at the respective output sides of the transfer registers 202R, 202G, and 202B.
In the CCD linear sensor having the structure described above, two-phase transfer pulses φH1b and φH2b are applied to each transfer stage of the transfer registers 102o and 102e of the monochrome sensor 100, a transfer pulse φLHb is applied to the final transfer stage in the vicinity of the output parts 104o and 104e, and a read-out pulse φROG2 is applied to the read-out gates 103o and 103e. Thus, output signals Vout-odd and Vout-even are output from the output circuits 105o and 105e, respectively.
Also, two-phase transfer pulses φH1c and φH2c are applied to each transfer stage of the transfer registers 202R, 202G, and 202B of the color sensor 200, a transfer pulse φLHc is applied to the final transfer stage in the vicinity of the output parts 204R, 204B, and 204B, and a read-out pulse φROG1 is applied to the read-out gates 203R, 203G, and 203B. Thus, output signals Vout-R, Vout-G, and Vout-B are output from the output circuits 205R, 205G, and 205B, respectively.
FIG. 5 illustrates the timing relationship among each of the timing pulses. Usually, in order to simplify the driving system, the pulses φH1b and φH1c are produced by one pulse, and the pulses φH2b and φH2c are produced by another pulse. Thus, in the monochrome sensor 100, the signal charge of each pixel of the pixel line 101 is read separately into odd and even sides, that is, the transfer registers 102o and 102e. Consequently, the transfer speed of the transfer registers 102o and 102e is the same as that of the color registers 202R, 202G, and 202B, and the transfer time is half that of the color side.
Specifically, since the monochrome sensor 100 has two transfer registers 102o and 102e, thus one frame time is half that of the color sensor 200. Here, one frame time means the repetition period of the read-out pulses φROG1 and φROG2. In the monochrome sensor 100, since one frame time is half that of the color sensor 200, it is possible to perform a high-speed read operation, and two read/transfer operations are possible during the period for one read/transfer operation by the color sensor 200, thus the resolution of the monochrome sensor 100 in the sub-scanning direction (in the direction perpendicular to the pixel line 101) can be twice that of the color sensor.
However, as described above, in the CCD linear sensor which includes two groups of sensors, that is, the sensors 100 and 200 having different transfer speeds, as is apparent from the timing chart in FIG. 5, during the transfer period of signal charge in the color sensor 200, a read operation of signal charge is performed in the monochrome sensor 100. Thus when the sensors 100 and the sensors 200 are located close to each other, in particular, when the sensors 100 and the sensors 200 are mounted on the same chip, there is a possibility that noise might be added to the pixel signal of the color sensor by the influence of the read-out pulse φROG2 when the pulse is generated.
In order to avoid the above, it is necessary to supply the transfer pulses φH1b and φH2b of the monochrome side, and the transfer pulses φH1c and φH2c of the color side with separate timings. In this case, the structure of the driving system, such as the timing generator for producing the transfer pulses φH1b and φH2b and the transfer pulses φH1c and φH2c, becomes complicated. Moreover, this results in increased cost.